In general, the optical fiber preform is produced by modified chemical vapor deposition (MCVD), outside vapor deposition (OVD) or vapor axial deposition (VAD) in which the preform is directly grown on the tip of a quartz bar.
The MCVD, the most widely used one of these methods, forms an optical fiber preform by depositing a core layer and a clad layer having different refractive indices in a quartz tube of a high purity. Here, the refractive indices are controlled by adjusting composition rates of deposited substances. That is, the refractive indices of the core and clad layers are controlled by differently adjusting the deposition rate of GeO2 to SiO2 in the composition used for the deposition of the core and clad layers.
The composition is usually supplied into the quartz tube by carrier gases and deposited while the outer surface of the quartz tube is heated to a temperature between 1300 to 1700° C. by a heat source such as gas torch. The gaseous reactant becomes the soot of SiO2 and GeO2 at such a high temperature.
While the soot of SiO2 and GeO2 passes through the quartz tube in the axial direction of the tube, a part of the soot is deposited on the wall of the quartz tube by thermophoresis and the rest thereof is discharged to the outside through a soot exhaust tube. The quartz tube with the sooty substance deposited on the wall is subjected to a collapsing process to produce a preform rod in the form of a cylinder and then a closing process to complete the entire process, thereby producing an optical fiber preform. Here, the quartz tube which is used to form optical fiber preform is called a substrate tube.
In recent years, there is a trend toward increased used of a horizontal lathe in the deposition process. A primary preform prepared in the deposition process using the horizontal lathe is subjected to the collapsing process and the closing process to form an optical fiber preform.
FIG. 1 is a schematic view of a conventional apparatus for producing an optical fiber preform in which deposition process according to the modified chemical vapor deposition method is performed.
As shown in FIG. 1, the conventional apparatus for producing an optical fiber preform includes a substrate tube 10, chucks 20 for holding hot ends of the substrate tube 10, supporting portions 30 for supporting respective chucks, a horizontal lathe 40 on which the support portions 30 are fixedly mounted, a main heat source 50 disposed on the horizontal lathe 40 between the supporting parts for beating substrate tube 10, a gas supply device 60 disposed outside the horizontal lathe for supplying gases to one end of the substrate tube 10, and a soot collector 70 disposed outside the horizontal lathe for collecting soot discharged Through a soot exhaust tube connected to the other end of the substrate tube.
Since the methods for producing an optical fiber preform by the conventional apparatus for producing an optical fiber preform having the above construction involve a high temperature deposition process, the mechanical strength and viscosity of the substrate tube 10 are reduced. Thus, in the process for producing a preform, the deposition should be carried out at a uniform rate and the outside diameter of the substrate tube 10 should be kept uniform while minimizing the deformation of the substrate tube 10 in the radial direction. However, the optical fiber preform produced by the conventional methods has elastic deformation and viscous deformation at the same time. That is, the finally produced substrate tube 10 deflects in proportion to the elastic deformation level, deposition time and viscous deformation level.
FIG. 2 is a schematic view showing the deflection phenomenon of the substrate tube 10 produced by the conventional apparatus for producing an optical fiber preform, wherein the substrate tube deflects more when the deposition process is carried out repeatedly.
Moreover, the substrate tube and the deposition time should be longer with a higher temperature for mass production, and it makes deflection phenomenon of the substrate tube 10 to be severer. Thus, when the deflection of the substrate tube 10 is excessive, the production of an optical fiber preform itself is impossible, or the produced optical fiber preform, where it may be produced, cannot properly shows its function.